Interferons (IFNs) are a family of cytokines that have been used as therapeutic agents against human malignancies. IFNs initiate a pleiotrophic array of cellular responses that may include the inhibition of cell growth and/or the induction of programmed cell death. While IFNs have shown success against a variety of malignancies, there have also been many failures. Scientists and physicians are, as of yet, unable to predict whether IFNs will be successful or unsuccessful in treating malignancies, in part because the mechanisms of how IFNs inhibit cell growth and/or promote apoptosis are poorly understood. We have recently begun characterizing a novel IFN-induced protein, hGBP-1, which is a human member of the interferon-induced family of GTPases, the guanylate binding proteins (GBPs). These are unique 65-67 kDa GTPases that hydrolyze GTP to both GDP and GMP. hGBP-1 is strongly anti-proliferative when expressed in some cancer cell lines and in endothelial ceils. To elucidate the mechanism(s) of hGBP-l-mediated inhibition of cell proliferation, the expression levels and activities of proteins involved in cell proliferation and the regulation of cell cycle progression will be examined subsequent to hGBP-1 expression. IFNs do not induce an antiproliferative response in a number of cancer cell lines. Whether or not the inability to respond to the antiproliferative activity of IFNs results from defects in the induction of hGBP-1 will be examined. Once the mechanisms of hGBP-1 action have been elucidated then we can also determine whether the carcinogenic process in unresponsive cells results in alterations in these pathways. In addition, the regions of hGBP-1responsible for its antiproliferative activity will be examined. To do this, mutant versions of hGBP-1 will be generated and evaluated for their anti-proliferative activity, cellular location, and ability to dimerize with wild type hGBP- 1. Understanding how hGBP-1 functions would make a significant contribution to understanding how IFNs inhibit cell growth.